1. Field of the Invention
The present invention relates to a magnetoresistance effect device, and also relates to a magnetic head, a memory device, and an amplifying device using such a magnetoresistance effect device.
2. Description of the Related Art
A magnetoresistive sensor (hereinafter referred to as an MR sensor) and a magnetoresistive head (hereinafter referred to as an MR head) using a magnetoresistance effect device have been under development. The term "a magnetoresistance effect element" indicates a device which varies an electric resistance depending on the magnetic field externally applied. The characteristic of the magnetoresistance effect device is generally represented by a ratio of change of magnetoresistance (hereinafter abbreviated as an MR ratio). The MR ratio is defined by the following equation: EQU MR ratio(%)=(R(maximum)-R(minimum))/R(minimum).times.100,
where R(maximum) and R(minimum) denote the maximum value and the minimum value of the resistance of the magnetoresistance effect device when a magnetic field is applied to the magnetoresistance effect device. Conventionally, as a material for a magnetoresistance effect device, a permalloy of Ni.sub.0.8 Fe.sub.0.2 is mainly used as the magnetic body. In the case of such magnetoresistance effect materials, the MR ratio is about 2.5%. In order to develop an MR sensor and an MR head with higher sensitivity, a magnetoresistive device exhibiting a higher MR ratio is required. It was recently found that [Fe/Cr] and [Co/Ru] artificial multilayers in which anti-ferromagnetic coupling is attained via a metal non-magnetic thin film such as Cr and Ru exhibit a giant magnetoresistance (GMR) effect in a ferromagnetic field (1 to 10 kOe) (see Physical Review Letter Vol. 61, p. 2472, 1988; and Physical Review Letter Vol. 64, p. 2304, 1990). However, these artificial multilayers require a magnetic field of several kilooersteds to several tens of kOe in order to attain a large MR change, so that such artificial multilayers cannot be practically used for a magnetic head and the like.
It was also found that an [Ni--Fe/Cu/Co] artificial multilayer using magnetic thin films of Ni--Fe and Co having different coercive forces in which they are separated by a metal non-magnetic thin film of Cu and are not magnetically coupled exhibits a GMR effect, and an artificial multilayer which has an MR ratio of 8% when a magnetic field of 0.5 kOe is applied at room temperature has been hitherto obtained (see Journal of Physical Society of Japan, Vol. 59, p. 3061, 1990). However, as is seen from the typical MR curve of this type of artificial multilayer shown in FIG. 11, a magnetic field of about 100 Oe is required for attaining a large MR change. Moreover, the MR asymmetrically varies from the negative magnetic field to the positive magnetic field, i.e., the MR exhibits poor linearity. That is, such an artificial multilayer has characteristics which are difficult to use in practice.
Also, it was found that [Ni--Fe--Co/Cu/Co] and [Ni--Fe--Co/Cu] artificial multilayers using magnetic thin films of Ni--Fe--Co and Co in which RKKY-type anti-ferro-magnetic coupling is attained via Cu exhibit a GMR effect, and an artificial multilayer which has an MR ratio of about 15% when a magnetic field of 0.5 kOe is applied at room temperature has been hitherto obtained (see Technical Report by THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS of Japan, MR91-9). As is seen from the typical MR curve of this type of artificial multilayer shown in FIG. 12, the MR substantially linearly varies from zero to the positive magnetic field, so that the film has characteristics which are sufficient for the application to an MR sensor. However, in order to get a large MR change, a magnetic field of about 50 Oe is required. Such a characteristic of the film is not appropriate for the application to an MR magnetic head which is required to be operated at most at 20 Oe and preferably less.
As a film which can be operated in a very weak magnetic field to be applied, a spin-valve type film in which Fe--Mn as an anti-ferromagnetic material is attached to Ni--Fe/Cu/Ni--Fe has been proposed (see Journal of Magnetism and Magnetic Materials 93, p. 101, 1991). As is seen from the typical MR curve of this type shown in FIG. 13, the operating magnetic field is actually weak, and a good linearity is observed. However, the MR ratio is as small as about 2%, and the Fe--Mn film has a poor corrosion resistance. The Fe--Mn film has a low Neel temperature, so that the device characteristics disadvantageously have great temperature dependence.
On the other hand, as a memory device using a magnetoresistance effect, a memory device using a conductor portion (sense lines) made of Ni--Fe(--Co)/TaN/Ni--Fe(--Co) in which Ni--Fe or Ni--Fe--Co as a conventional MR (magnetoresistance effect) material is laminated via TaN has been proposed (see U.S. Pat. No. 4,754,431, and IEEE Trans. Magn. Vol. 27, No. 6, 1991, pp. 5520-5522). Such a memory device utilizes the conventional material as an MR material, so that the MR ratio is 2% to 3%. Thus, the memory device has disadvantages in that the output during the information read-out is weak, and it is inherently difficult to perform nondestructive read-out.